In recent years, attention has been paid to semiconductor devices using a group III-V compound having a band gap larger than that of Si. Among them, development of a semiconductor device using gallium nitride (GaN) which has such advantages as 1) large breakdown field, 2) large electron saturation rate, 3) large thermal conductivity, 4) capability for formation of excellent heterojunction between AlGaN and GaN, 5) non-toxicity and high safety, and the like has been advanced.
Further, in view of high withstand voltage and high-speed switch characteristics, development of a semiconductor device capable of normally-off operation such as a power MISFET (Metal Insulator Semiconductor Field Effect Transistor) using gallium nitride has been advanced.
For example, Japanese Patent Application Laid-Open Publication No. 2011-9493 (Patent Document 1) discloses a normally-off type nitride semiconductor device having a recess structure in which an AlGaN layer other than a gate region is made thick by a selective regrowth using an epitaxial growth method. In this nitride semiconductor device, a high-concentration doped layer (5) or a planar doped layer (52) is provided at an interface having many trap levels between an epitaxial growth layer and a selective regrowth layer.
Also, International Patent Publication No. 2009/113612 (Patent Document 2) discloses a semiconductor device in which a lower barrier layer made of lattice-relaxed AlxGa1-xN (0≦x≦1), a channel layer made of InyGa1-yN (0≦x≦1) having a compressive strain, and a contact layer made of AlzGa1-zN (0≦x≦1) are sequentially stacked. Two dimensional electron gas is produced in the vicinity of an interface between the InyGa1-yN channel layer and the AlzGa1-zN contact layer.
Also, Japanese Patent Application Laid-Open Publication No. 2010-272728 (Patent Document 3) discloses a GaN-based semiconductor element (1) in which an AlN layer (12), a buffer layer (13) formed by stacking GaN layers and AlN layers alternately, and a channel layer (14) composed of a p-GaN layer are formed on a substrate (11) made of sapphire, SiC, Si or the like. On the channel layer (14), an electron transit layer (15) made of undoped GaN (un-GaN) and an electron supply layer (16) made of GaN-based semiconductor (AlGaN) having a band gap energy larger than that of the electron transit layer (15) are sequentially stacked. Further, portions (gate-electrode forming region) of the electron transit layer (15) and the electron supply layer (16) are removed to a depth reaching the channel layer (14), so that a recess portion (18) is formed.
Also, Japanese Patent Application Laid-Open Publication No. 2011-243978 (Patent Document 4) discloses a nitride-based semiconductor element (10) composed of a substrate (12), a buffer layer (14), a GaN layer (16), an AlGaN layer (20), a gate insulating film (22), a source electrode (24), a gate electrode (28), a drain electrode (26), and an SBD metal electrode (30) (see FIG. 1). Further, the Patent Document 4 discloses a nitride-based semiconductor element (80) in which an n+ AlGaN layer (83-1) and an n+ GaN layer (82-1) on the source electrode (24) side and an n+AlGaN layer (83-2) and an n+ GaN layer (82-2) on the SBD metal electrode (30) side constituting a first n+ region are provided in a region below the gate insulating film (22) (see FIG. 11).
Also, N. Ikeda et al. “Over 1.7 kV normally-off GaN hybrid MOS-HFETs with a lower on-resistance on a Si substrate”, IE3 International Symposium on Power semiconductor Devices and ICs (ISPSD), pp. 284-287, 2011 (Non-Patent Document 1) discloses a MISFET with a structure in which, in order to realize a normally-off operation by using a heterojunction between AlGaN and GaN, a gate recess is dug toward a back side beyond the heterojunction. Further, N. G. Weinmann et al., JOURNAL OF APPLIED PHYSICS, VOLUME 83, NUMBER 7, P. 3656, 1998 (Non-Patent Document 2) discloses a relationship between a doping amount into a GaN substrate and an electron mobility.
Incidentally, reference numerals in parentheses indicate reference numerals in the respective Patent Documents.